The present invention is generally related to optical communications systems, and more particularly to a method of limiting polarization-mode dispersion (PMD) in the optical link including reducing unnecessary corrective actions based upon a limit condition declared by a PMD compensator.
In a typical optical communications system, an optical signal is emitted from a modulated laser diode comprising an optical transmitter in the form of a series of light pulses. Each light pulse is of extremely short duration, such as 40 ps, and is roughly Gaussian shaped as a function of time. In the frequency domain, this signal comprises numerous frequency components spaced very closely about the nominal center frequency of the optical carrier such as 193,000 GHz. As this type of modulated optical signal passes through an optical fiber, different frequency components of the optical signal travel at slightly different speeds due to an effect known as chromatic dispersion. In the course of an optical signal traveling through a very long fiber, such as 200 km, chromatic dispersion causes a single pulse of light to broaden in the time domain, and causes adjacent pulses to overlap one another, interfering with accurate reception. Fortunately, many techniques are known for compensating for chromatic dispersion.
Another form of dispersion is becoming a limiting factor in optical communications systems as progressively higher data rates are attempted. Polarization-mode dispersion (PMD) arises due to birefringence in the optical fiber. This means that for two orthogonal directions of polarization, a given fiber can exhibit differing propagation speeds. A light pulse traveling through a fiber will probably, unless some control means are employed, have its energy partitioned into polarization components that travel at different speeds. As with chromatic dispersion, this speed difference causes pulse broadening and restricts the usable bandwidth of each optical carrier.
A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Conventionally, a properly designed optical link can maintain a bit-error-rate (BER) of 10xe2x88x9213 or better. Noise, attenuation, and dispersion are a few of the impairments that can render an optical signal marginal or unusable at the receiver. Generally, when an optical channel degrades to a bit-error-rate of 10xe2x88x928, a communications system will automatically switch to an alternate optical channel in an attempt to improve the BER.
One common method of analyzing the quality of a modulated optical signal is a so-called xe2x80x9ceye diagramxe2x80x9d, shown in FIG. 1. The eye diagram consists of overlaying successive frames of time-domain traces of the signal, with each frame corresponding to one period of the nominal periodicity of the modulation. As portrayed, the vertical axis represents instantaneous intensity of the received signal, and the horizontal axis corresponds to time. Many successive traces of transmitted xe2x80x9conesxe2x80x9d and xe2x80x9czerosxe2x80x9d define a region or window within the middle of the display. In the time axis, the window is bound on either side by the transitional leading and trailing edges of the pulses. Using this technique, a large clear area or xe2x80x9cwindowxe2x80x9d in the center with no encroachment from any side represents a good signal in that the presence of absence of a pulse during each clock cycle is clearly distinguishable.
Noise added to a signal appears as xe2x80x9cfuzzinessxe2x80x9d of the lines defining the window. Sufficient noise can even obliterate the appearance of the window, representing a bad signal in that xe2x80x9conesxe2x80x9d and xe2x80x9czerosxe2x80x9d are no longer distinguishable. Impairments in the time axis, such as chromatic dispersion or polarization mode dispersion, cause the transitional areas of the display to close in upon the window from either side. Overlapping of pulses can require more stringent synchronization of the receiver""s decision point, or even render the signal unusable.
A given optical receiver will automatically adapt to receive a modulated optical signal. Automatic gain control (AGC), frequency control, and phase lock-in are typically applied in sequence so that a threshold decision circuit can best sample the signal and decode every pulse. Superimposed upon the eye-diagram, an optimal point of operation for a threshold decision circuit would intuitively be at the center of the window, as shown by the xe2x80x9c+xe2x80x9d in FIG. 1. This means that the intensity threshold is about halfway between the zero values and one values observed on average.
Timewise, the center of the window corresponds to sampling the pulses in the middle of their duration when they tend to be of maximum intensity also shown by the xe2x80x9c+xe2x80x9d in FIG. 1. Intuitively, one can see how this choice for an operating point would be the most robust against either noise or timing impairments which cause the window to shrink.
A received optical signal can undergo some degree of change in, for example, pulse width without having an immediate impact on BER as observed by this optimally positioned main decision circuit. A particular type of receiver has been developed comprising at least two independent decision circuits of the type just described. A main decision circuit is dedicated to actual communications reception and is maintained at the optimum point, once it is established, within the center of the window. But for analyzing signal quality to a finer degree and for measuring degradation before it impacts BER of the actual communications, an auxiliary decision circuit is used to probe the extents of the operating window. Robustness to timewise disturbances is gaged by directing the auxiliary decision circuit to sample at various time offsets relative to the optimum point. Findings by the auxiliary circuit may even be used to fine-tune the optimum point settings of the main decision circuit.
The auxiliary decision circuit is set to a given timing offset and its output is monitored for BER, especially in comparison to the output of the main decision circuit. The BER measurement at each operating point can typically take several minutes. Gradually, BER data is accumulated for every offset value. As expected, a plot of this data resembles an inverted Gaussian curve with a minimum BER occurring some optimum offset, as shown in FIG. 2. A similar plot is derived by varying the amplitude threshold of the auxiliary decision circuit.
All of this BER data may be summarized into a xe2x80x9cQxe2x80x9d factor or quality metric for the received signal. In general terms, the broader the range of timings over which a low BER can be sustained, the greater the Q factor of the signal. A receiver with an auxiliary decision circuit can measure and output such a Q factor.
During the time that an auxiliary decision circuit is accumulating measurements to compile a Q factor for a received signal, a shift in dispersion characteristics, particularly PMD characteristics, can take place along the fiber. This can result in an inaccurate assessment of the signal quality, especially if a PMD controller (PMDC) cannot quickly and sufficiently compensate for the PMD change. Therefore, the Q factor cannot be solely relied upon as a measure of path quality.
The Q measurement is particularly useful for assessing and fine-tuning an optical path that is already operating at a healthy low BER. The Q measurement estimates a BER without requiring any actual bit errors to occur. A Q measurement covering the BER range of 10xe2x88x9213 to 10xe2x88x9220 may be completed in a few minutes, whereas an actual errored bit might not be observed for hours, days or months.
Schemes to actively compensate for PMD generally involve detecting the presence of polarization-dependent timing differences and either a) applying delay elements to one or the other polarization to realign the timing of pulses or b) controlling the polarization state of the signal upon entry into the fiber, or at intermediate points along the fiber, such that birefringent effects are minimized or canceled out. Active compensation techniques are required because the PMD of a given fiber varies over time due to temperature and pressure changes along the fiber, and due to aging. A fiber that is installed above ground can exhibit fairly rapid fluctuations in PMD due to temperature and mechanical forces. A fiber buried underground can be sensitive to loads such as street traffic or construction work.
PMD of the optical carrier along the optical fiber may be reduced by the placement of one or more state-of-polarization (SOP) controllers along the fiber. A SOP controller can output light at one of several selectable polarization states. For optical communications, the relative rotation angle is the main property that is controlled. By adjusting the state-of-polarization of an optical carrier as it enters the fiber, the PMD effects of the fiber can be minimized. Furthermore, SOP controllers applied at several points along a fiber can effectively use the fiber""s own characteristics to cancel PMD.
A common PMD controlled optical path comprises a transmitter, a fiber with one or several SOP controllers, a PMD controller (PMDC) and a receiver. For a modulated optical signal, the PMDC can sense the timing difference between a pair of orthogonal polarizations and selectively delay one polarization to realign the timing between the two signal halves before passing the signal to the receiver. The receiver expects a signal with less than a certain amount of chromatic and polarization-mode dispersion. As the polarization characteristics of the fiber change, the PMDC constantly monitors the signal and adjusts to minimize the PMD contribution to overall dispersion.
A problem arises in that the PMDC typically has a finite range of delay compensation that it can accomplish. Normally, the SOPs along a fiber are set such that, on average, the bulk of the PMD is curtailed before the PMDC. However, on some occasions, the PMD of a given fiber can drift beyond the range of the PMDC.
When the PMD of the optical path degrades, some corrective action may be necessary either to improve the PMD of the optical path or to divert the communications traffic along an alternate channel or path that will work better. Yet, it is equally important to the integrity of the traffic bearing signal to avoid taking unnecessary corrective actions. Each adjustment or switching operation can temporarily disrupt the revenue bearing traffic signal.
There is desired a way to quickly restore the integrity of an optical link under these conditions, including improving PMD, while reducing corrective actions based upon limit conditions declared by a PMDC.
The present invention achieves technical advantages as an optical communication system having a SOP controller along an optical link that is intelligently and responsively directed to change rotation states whenever a PMDC surpasses a preset compensation limit. The SOP controller is initially set such that the likelihood of the PMDC reaching a compensation limit is acceptably small. Upon the rare occasion that the compensation limit is exceeded, one or more of the SOP controllers are commanded to alter rotation of polarization by some arbitrary amount, for example, 45 degrees to 90 degrees. This rotation action will most likely result in lowering the end-to-end PMD to acceptable levels. The rare condition of high PMD occurs when the fiber shifts properties and exhibits a worst case dispersion in conjunction with the settings of the SOP controllers. Under worst conditions, changing any of the SOP controllers in either direction will most likely result in reduced PMD.
According to the present invention, since this rotation action can cause a momentary disruption in the traffic-bearing optical signal, abrupt changes in SOP controllers are intelligently made along an optical link only when absolutely necessary to preserve the integrity of the link. The present invention utilizes intelligent fault detection processing of the optical system to provide a safeguard against false triggering and making unnecessary adjustments to a SOP controller. A controller intelligently processes several indicators from system devices, including notifications from the PMDCs, to determine whether the limit condition declared by a PMDC is the main problem affecting the link, or if the limit condition is simply a consequence of a separate problem that the PMDC should not try to fix.
The present invention correlates several indicators to intelligently generate the switch rotation state indication for the SOP controller, including notifications from PMDCs, a detected BER, Q factor and signal-to-noise ratio (SNR), to decide whether a path degradation is truly attributable to PMD or other causes. The present invention provides a technique to gate whether or not the SOP controller should abruptly shift states.